Dee Bee What - Know the difference
dB should be used when comparing 2 different output powers or adding to a known power level. For example
"This antenna provides 2dB more gain than this other antenna"
It is a relative term which means that you use it when there is another output power to relate to.
You cannot for example state
"My radio outputs 20dB of Power"
To do so would lose you a very large number of "geek points"
dB is a logarithmic measurement which means that every 3dB you add to an output power doubles the total output power and every 3dB you subtract from an output power halves the total output power. Adding 10dB increases the total output power by 10 times and deducting 10dB decreases the total output power by the same
dBm is used when you want to reference the output power of a radio. It is a known scale (with 0dBm being equal to 1mW). For example
"This radio outputs a whopping 50dBm and thus Ofcom are likely to come round and beat you with a large stick"*
dBm also follows the same logarithmic scale as dB so knowing that 0dBm is 1mW we know that 3dBm is 2mW and 6dBm is 4mW etc. The "m"; indicates that it is a reference against a milliwatt reading.
dBi is most often used when referencing Antenna gain (note this is gain rather than output power as the antenna itself has no output power it just increases the output power of the radio). For example
"I have a 20dBm radio and I add a 10dBi Antenna to it, this gives me 30dBm of total output power"
dBi is referenced to an Isotopic source which is a theoretic perfect omnidirectional antenna (an example of this is the Sun). It is used as it is a constant value that can be used for comparisons. The "I" indicates that it is a reference against an isotropic antenna.
dBd is used when comparing the amount of gain an antenna has against a Diapole antenna. A diapole antenna has a gain 2.15 dBi. This means that a 0dBd gain antenna has 2.15dBi of gain
*Please note, as far as I am aware it is not common practice for Ofcom to beat people with sticks